CDK4 and breast cancer: Among the large variety of possible candidates as predictive biomarkers to explore, TK1 has been of particular interest for us since it has been recently shown that high TK1 activity and expression levels in the serum of BC patients are associated with a higher risk of recurrence [30,31], PFS, and OS [32], and that its overexpression in plasma-derived exosomes is associated with clinical resistance to CDK4/6 inhibitors in patients with metastatic BC [33].